Production and use of octafluoropropane

Abstract

Octafluoropropane is produced by a process comprising a step (1) of reacting hexafluoropropene with hydrogen fluoride in a gas phase at a temperature of from 150 to 450° C. in the presence of a fluorination catalyst to obtain 2H-heptafluoropropane and a step (2) of reacting 2H-heptafluoropropane obtained in step (1) with fluorine gas in a gas phase at a temperature of from 250 to 500° C. in the absence of a catalyst to obtain octafluoropropane. High-purity octafluoropropane is obtained which can be used in a process for producing a semiconductor device.

Description

The present invention relates to a process for producing octafluoropropane, an octafluoropropane product, and uses thereof.Octafluoropropane is used, for example, as a dry-etching or cleaning gas in the process for producing a semiconductor device. With respect to the production methods thereof, the following methods are known:(1) a method of performing a direct fluorination reaction between hexafluoropropene and fluorine gas (see, Japanese Examined Patent Publication No. 62-61572 (JP-B-62-61572)),(2) a method of performing an electrolytic fluorination of hexafluoropropene in hydrogen fluoride (see, Japanese Examined Patent Publication No. 62-61115 (JP-B-62-61115)),(3) a method of reacting hexafluoropropene with fluorine in the presence of a catalyst (see, Japanese Examined Patent Publication No. 1-45455 (JP-B-1-45455)), and(4) a method of reacting hexafluoropropene with a high-order metal fluoride (see, Japanese Examined Patent Publication No. 62-54777 (JP-B-62-54777)).However, in ...

AI Technical Summary

Benefits of technology

In the process for producing octafluoropropane of the present invention (I), a step (1) of reacting hexafluoropropene with hydrogen fluoride in a gas phase at a temperature of from 150 to 450.degree. C. in the presence of a fluorination catalyst to obtain 2H-heptafluoropropane is first performed. Step (1) has the following two advantageous points.

[2] As described above, hexafluoropropene contains compounds having chlorine atom within the molecule as impurities in many cases and these compounds are difficult to remove by a distillation operation. In the present invention, at the same time as the reaction of adding hydrogen fluoride to hexafluoropropene to obtain 2H-heptafluoropropane as an intermediate, compounds having a chlorine atom within the molecule are fluorinated using the hydrogen fluoride and thereby converted into compounds which are easy to separate by distillation.

The fluorination catalyst may be a commonly used chromium-based catalyst. In the case where hexafluoropropene contains chlorine-containing impurities and the chlorine-containing impurities are fluorinated and thereby converted into other compounds, the reaction temperature becomes higher, therefore, a bulk catalyst mainly comprising an oxide of chromium and obtained by adding at least one member selected from indium, zinc and nickel is preferably used, because this catalyst has excellent activity (performance) and high stability (catalyst life). A supported catalyst (e.g. on an alumina support) may also be used, however, a bulk catalyst is preferred in view of activity and stability. Such a catalyst may be activated by a fluorination treatment with hydrogen fluoride before use in the reaction.

As is apparent from Table 2, there is obviously a large difference in boiling point between 2H-heptafluoropropane as an intermediate and the compounds fluorinated according to the above-described reactions, as a result, separation of these compounds by distillation is facilitated.

The step (2) is a direct fluorination reaction step of reacting 2H-heptafluoropropane obtained in the fluorination step of the step (1) with fluorine gas in the gas phase at a temperature of from 250 to 500.degree. C. in the absence of a catalyst to obtain octafluoropropane. This step has the following three advantageous points.

[3] In the direct fluorination reaction of reacting 2H-heptafluoropropane with fluorine gas, if 2H-heptafluoropropane is used in excess based on the fluorine gas, a step of removing fluorine gas may be dispensed with but great difficulties are incurred in the later separation and purification. In the step (2) of the present invention (I), the fluorine gas can be used in an excess molar amount based on 2H-heptafluoropropane so as to elevate the reaction efficiency and when fluorine gas is used in excess, the reaction production gas outflowing from the reaction step contains the excess fluorine gas in addition to perfluorocarbon and hydrogen fluoride. For treating the excess fluorine gas, a method of reacting the gas with an inorganic oxide such as alumina or soda lime is known, however, this method is not preferred because water is produced by the reaction and causes corrosion of apparatus materials. In the present invention (I), the excess fluorine gas can be removed by contacting the gas with hydrofluorocarbon at 1.1 molar times, in terms of the chemical equivalent ratio, to the excess fluorine gas.

Problems solved by technology

However, in these methods, by-products such as tetrafluoromethane (CF.sub.4) and hexafluoroethane (C.sub.2 F.sub.6) are produced due to cleavage, C.sub.6 F.sub.12 and C.sub.6 F.sub.14 are produced due to radical addition, and a 4-membered ring is produced due to cyclization addition, for example, and as a result, the yield of and selectivity for the objective octafluoropropane decrease.

Furthermore, some compounds in these impurities are difficult to separate by distillation and, in turn, high-purity octafluoropropane can hardly be obtained.

Particularly, in the case of using hexafluoropropene as the starting material, chloropentafluoroethane (CFC-115) contained as an impurity scarcely reacts with fluorine gas and mostly remains in the objective octafluoropropane and since this impurity compound can hardly be separated by distillation, due to the similar boiling points, production of high-purity octafluoropropane is difficult.